Add an optional GimpTransformGridTool::radjust() virtual function,
which subclasses can implement to radjust the transformation based
on the current state of the display, such that it's easy to
control.  This is especially useful when the image is zoomed-in,
and the transform handles, which are initially across the layer
bounds, are out of view.

When a transform tool implements radjust(), show a "Readjust"
button in the tool GUI.  While readjusting the transformation, we
modify the opposite transformation such that the overall transform
remains unchanged, as if both transform-directions were linked, so
that only the transform grid is readjusted.